a11y(transcription): give @mention search input its own sr-only label

The sr-only label for the search input was reusing the listbox
"Link person" label — but the input filters a candidate list, it does
not link anything. Screen readers heard a verb mismatch between the
listbox announce and the search-input focus event. New
person_mention_search_label key in de/en/es. The listbox aria-label
stays person_mention_btn_label since that labels the listbox itself.
Leonie FINDING-MENTION-004 on PR #629.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/de.json

@@ -445,6 +445,7 @@
 	"person_mention_load_error": "Person konnte nicht geladen werden.",
 	"person_mention_loading": "Lade Person…",
 	"person_mention_popup_empty": "Keine Personen gefunden",
+	"person_mention_search_label": "Person suchen",
 	"person_mention_search_prompt": "Namen eingeben…",
 	"person_mention_btn_label": "Person verlinken",
 	"person_mention_create_new": "Neue Person anlegen",

frontend/messages/en.json

@@ -445,6 +445,7 @@
 	"person_mention_load_error": "Could not load person.",
 	"person_mention_loading": "Loading person…",
 	"person_mention_popup_empty": "No persons found",
+	"person_mention_search_label": "Search persons",
 	"person_mention_search_prompt": "Enter a name…",
 	"person_mention_btn_label": "Link person",
 	"person_mention_create_new": "Create new person",

frontend/messages/es.json

@@ -445,6 +445,7 @@
 	"person_mention_load_error": "No se pudo cargar la persona.",
 	"person_mention_loading": "Cargando persona…",
 	"person_mention_popup_empty": "No se encontraron personas",
+	"person_mention_search_label": "Buscar persona",
 	"person_mention_search_prompt": "Escribe un nombre…",
 	"person_mention_btn_label": "Vincular persona",
 	"person_mention_create_new": "Crear nueva persona",